Plant-parasitic nematodes worldwide cause diseases of nearly all crop plants of economic importance with estimated losses of about $ 5.8 billion/yr in the Unites States alone (Sasser and Freckman, 1987, World prospective on nematology In: Vistas on Nematology, Eds. Veech & Dickson. Hyatts Will, Md. pp. 7-14). While in tropical regions losses caused by nematodes are due mainly to root-knot nematodes (Meloidogyne), in Europe cyst nematodes of the genera Globodera and Heterodera are regarded as serious pests and important limiting factors in e.g. potato, rapeseed and sugarbeet cultivation, respectively. Only a small number of resistant crop varieties have emerged from breeding programmes for e.g. potato, sugarbeet, tomato, soybean and oil radish (Dropkin, 1988, Ann. Rev. Phytopath. 26, 145-161; Trudgill, 1991, Ann. Rev. Phytopath. 29, 167-192). The resistance is often based on single R-genes (Rick & Fobes, 1974, Tomato Gen. Coop. 24, 25; Barone et al. 1990, Mol. Gen. Genet. 224, 177-182) and leads to breakdown of resistance after several generations (Shidu & Webster, in: Plant Parasitic Nematodes, Vol. III, 1981, Zuckerman et al. (eds.) Acad. Press, New York, pp 61-87; Turner, 1990, Ann. Appl. Biol. 117, 385-397).
Plant-parasitic nematodes are obligate parasites. Nematodes such as cyst and root-knot nematodes are completely dependent on the formation of proper feeding structures inside the plant root. These feeding structures arise from single root cells that are selected by the nematode after invasion of the root. In the case of cyst nematodes, the IFC (initial feeding cell) develops into a syncytium through digestion of cell walls and hypertrophy. After infection with a root-knot nematode, the IFC develops into a giant cell through several nuclear divisions without cytokinesis and becomes metabolically very active. During establishment of the feeding structure, the infective juvenile nematode becomes immobile and loses the ability to move to other feeding sites, illustrating their dependance on the induced nematode feeding structure (NFS).
Clearly, there is a great need for plants with reduced susceptibility to plant parasitic nematodes. Current strategies to combat pathogens and pests involve expression of recombinant DNA encoding a product which has a direct interaction with the pathogen or pest.
EP-A 159 884 teaches the expression in plants of a gene encoding an insecticidal toxin of Bacillus thuringiensis. Once the protein is digested by the insect it binds to a receptor in the gastrointestinal tract, eventually resulting in the death of the insect. The interaction of the toxin and a receptor in the insect is crucial to the toxic effect, which may explain the relatively frequent instances of acquired resistance to the toxin.
EP-A 303 426 reports that a number of B. thuringiensis strains act on nematodes. It is therefore not surprising that the suggestion is made to identify the genes that encode for these nematicidal toxins and express these genes in plants to protect them from nematodes (EP-A 352 052).
However, it is of some concern that the direct interaction between toxin and pest is prone to the swift development of resistance, as has been reported for B. thuringiensis endotoxin (Peferoen M. (1991) Agroindustry High-Tech , p. 5-9.
It is therefore an object of the present invention to provide plants with reduced susceptibility to pathogens and pests which avoids the problem of acquired resistance altogether.